David Gil's claim (as reported in Linguist 15.163) that Riau
Indonesian has no 'parts of speech' may or may not be
well-founded. However, its validity is a matter for empirical
investigation, and cannot be decided by theoretical fiat.
In her rejoinder (Linguist 15.163) Whitney Anne Postman argues,
effectively, that all languages must distinguish reference and
predication grammatically (and hence contain distinct 'noun' and
'verb' classes) because reference and predication are universal
semantic categories, fundamental to the way in which we perceive the
world and communicate about it. But 'part of speech' categories are
formal, not semantic entities, and cannot be posited to exist on the
basis of purely semantic considerations.
Conceptual universals do not necessarily find expression in the
grammars of all languages; they may be expressed lexically, or left to
be inferred pragmatically. Time and number are obvious cases in
point. The presence in Riau, or any other language, of a grammatical
counterpart of the reference-predication distinction can therefore
only be established by linguistic investigation. For 'nouns' and
'verbs' to exist as distinguishable word-classes in Riau, there must
be observable differences between them. If this is not the case, the
classes cannot be magicked into being by the unfalsifiable claim that
they exist at an ill-defined 'deeper level'.
-Michael Swan